Polarization entanglement with highly nondegenerate photon pairs enhanced by an effective walk-off-compensation method

We present an entangled photon source that produces polarization entanglement in highly nondegenerate photon pairs, generated through Type-0 spontaneous parametric down conversion using bulk periodically poled lithium niobate. This source is specifically designed for entanglement-based quantum key distribution between ground and satellite, as part of the Quantum Encryption and Science Satellite mission funded by the Canadian Space Agency. Through the utilization of birefringent beam displacers in a Sagnac interferometer scheme, we ensure high polarization contrast and stable interference of highly nondegenerate photon pairs. However, this approach causes substantial spatial and temporal walk-offs of the photon paths, posing a formidable challenge. We introduce an effective compensation method using birefringent crystal wedges to eliminate spatial and temporal walk-offs simultaneously. With only $1.0\phantom{\rule{0.16em}{0ex}}\mathrm{m}\mathrm{W}$ of pump power, we observed a coincidence rate of $(33.33\ifmmode\pm\else\textpm\fi{}0.05)\mathrm{kHz}$, a significant improvement compared to the rate of $(4.20\ifmmode\pm\else\textpm\fi{}0.01)\text{kHz}$ without spatial compensation. Additionally, we observed an estimated pair generation rate of $(2.92\ifmmode\pm\else\textpm\fi{}0.12)\text{MHz}$ and an entanglement visibility of $(97.1\ifmmode\pm\else\textpm\fi{}0.3)%$, making this a promising source for long-distance quantum communication in ground-to-satellite and fiber optic links.